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Why does the whiteness of the product decrease when the amount of fluorescent whitening agent is too large? Is there any standard dosage of various whitening agents?

2020-06-28

There are many kinds of fluorescent whitening agents, and all kinds of plastic whitening agents have their relatively suitable whitening agents. Although the chemical structure and function of various whitening agents are different, the whitening principle for products is the same.

The whitening principle is mainly due to the conjugated double bond system contained in the molecules of the whitening agent, which has excellent planarity. This special molecular structure can absorb ultraviolet rays (wavelength is 300-400 nm) in sunlight and reflect blue-purple light (wavelength is 420-500 nm), and the blue-purple light is mixed with yellow light on the product to become white light, thus the plastic is significantly whitened.




▲ Reflectance curve of samples containing whitening agent

According to the principle of fluorescent whitening agent, the whitening effect of whitening agent mainly depends on the content of ultraviolet rays in sunlight and the concentration of fluorescent whitening agent in products.

When the content of ultraviolet rays in sunlight is sufficient and the concentration of fluorescent whitening agent on the product surface changes within a certain scale, its whitening effect increases with the addition of the concentration of whitening agent.

However, when the concentration of whitening agent is increased to a certain appropriate concentration, its whitening effect is the best and the highest whiteness value can be obtained. If the dosage of whitening agent exceeds the optimal concentration, it will not only not improve the whitening effect of the product, but will instead decrease (i.e. the yellowing of plastic products is generally referred to as the yellowing point of whitening agent). At this moment, the optimal concentration of whitening agent is called the yellowing point of whitening agent.

Of course, the yellowing point of various fluorescent whitening agents is different, for example, the yellowing point of fluorescent whitening agent DT (for polyester) is 0 .8%. That is, when the amount of DT whitening agent exceeds 0 .8%, the whiteness value decreases with the addition of concentration.



Then, the main reason why the whitening effect is no longer enhanced with the addition of fluorescent whitening agent concentration is that the strength of yellow light on the plastic surface is limited, so the complementary color light of yellow light emitted by fluorescent whitening agent due to absorption of ultraviolet rays (main wavelength is about 570nm)-blue-purple light required to offset the yellow light is limited.




As the concentration of fluorescent whitening agent is added, the intensity of blue and violet light emitted by the fluorescent whitening agent is also added, thus offsetting some yellow light on plastic products, gradually reducing the intensity of yellow light, and the plastic products become whiter and whiter.

When the concentration of fluorescent whitening agent on plastic products is added to a proper concentration (i.e. yellowing point of whitening agent), the intensity of blue-violet light emitted by the fluorescent whitening agent is just the same as that of yellow light on plastic products, which is just offset by each other. At this time, the product is also whitest and has the best whitening effect.

When the intensity of blue-violet light is greater than the intensity of yellow light on the product, the blue-violet light reflected by the whitening agent is extremely significant. At the same time, due to the different constituent groups of the whitening agent, the different colors (e.g., cyan, red, blue, etc.) are also significantly reflected. As a result of the inductive effect of the above two elements, the gray color on the surface of the product is added, and its effect is strengthened with the addition of the whitening agent concentration, thus reducing the whitening effect. Although there is no trace of yellow light on the product at this time, it no longer looks bright, bright and dazzling.

Therefore, when choosing whitening agents, besides considering their whitening effects, small sample experiments should be carried out to find yellowing spots of different whitening agents on plastic raw materials so as to make reasonable use and obtain the best whitening effect.